Effect of aging treatment on the precipitation transformation and age hardening of Mg-Gd-Y-Zn-Zr alloy

In this study, the precipitation transformation and age hardening of solution-treated Mg-9Gd-4Y-2Zn-0.5Zr(wt.%) alloy were investigated at different aging treatment parameters. The precipitation sequences of the alloy aged at 200℃, 250℃ and 300℃ are β’’(DO19) → β’(BCO) → β(FCC), β’’(DO19) → β’(BCO) → β_(1)(FCC) → β(FCC) and β(FCC), respectively. The streaks sequences of the alloy aged at 200℃, 250℃ and 300℃ are SF, SF → 14H-LPSO and SF → 14H-LPSO, respectively. For the alloy aged at 200℃ and 250℃, the increase in hardness with increasing aging time is contributed from the increase in precipitate volume fraction and the transformation from β’’ to β’ phase with basal → prismatic and spherical → spindle-like precipitate changes. The decrease in hardness after the peak-aging stage is attributed to the appearance of micro-sized β precipitates. Because of the smaller size of precipitates and the triangular arrangement of β’ precipitate, the hardness of the alloy aged at 200℃ is higher than that aged at 250℃. For the alloy aged at 300℃, the appearance of only micro-sized β precipitate and its coarsening with increasing aging time leads to the lowest hardness and an overall decrease in hardness with the aging time.

Age hardening,fracture behavior and mechanical properties of QE22 Mg alloy

The microstructure,mechanical properties and fracture behavior of an as-received QE22 alloy have been investigated under different thermal conditions,including solution treated(ST),under aged(UA),peak aged(PA)and over aged(OA)conditions.A significant increase in hardness of 27%,yield strength of 60%and ultimate tensile strength of 19%was observed in peak aged sample as compared to solution treated sample.The improvements of mechanical strength properties are mainly associated with the metastable λ and β′precipitates.Grain growth was not observed in the ST samples after subjecting to UA and PA treatments due to the presence of eutectic Mg_(12)Nd particles along the grain boundaries.In over aged sample,significant grain growth occurred because of dissolution of eutectic phase particles.Different natures of crack initiation and propagation were observed under different thermal conditions during tensile testing at room temperature.The mode of failure of solution treated sample is transgranular,cleavage and twin boundary fractures.A mixed mode of transgranular,intergranular,cleavage and twin boundary failure is observed in both peak aged and over aged samples.

Consequence of reinforced SiC particles and post process artificial ageing on microstructure and mechanical properties of friction stir processed AA7075

Friction stir processing and post process artificial ageing was successfully carried out on AA7075 with and without reinforcement of SiC particles producing defect free processed zone with uniform distribution of filler material.Effect of SiC particle reinforcement and artificial ageing times on the microstructural modifications was characterized using optical and electron microscopy,electron backscattered diffraction and X-Ray diffraction.Hardness,impact and wear tests were carried out to investigate mechanical behaviour before and after processing.Reinforcement of SiC particles during FSP and subsequent age hardening treatment brought about nearly twofold increase in hardness and impact toughness values by the combined effect of grain refinement,Zener pinning,dispersion strengthening and precipitation hardening.Significant improvement in wear resistance in terms of wear loss was also observed after processing compared to the reference material AA7075-T6.Fractured surface of post FSP age hardened AA7075 alloy exhibited features of ductile fracture during Charpy impact test.

Electrical Conductivity Behavior of the Aluminum Alloy 2024 during Artificial Aging

Precipitation-hardened aluminum alloys need to be heat treated to achieve the mechanical properties required for their application.The production of these materials can be optimized to make them more attractive and competitive comparing to other materials such as composites that have a growing and large market share in aeronautics field.One way to do this is by controlling the artificial aging of precipitation-hardenable aluminum alloys,such as the 2000 series Al-Cu alloys.These alloys can be monitored in real time by analyzing their conductivity behavior inside the furnace.The objective of this work is to evaluate the electrical conductivity behavior in real time of the 2024 alloy during the artificial aging at 190°C.For this,analyses were made in order to assess the behavior of the microhardness curve by aging time and its microstructural characterization with thermal treatments in the times of 1 h to 9 h interrupted every 1 h.The results of the electrical conductivity versus hardness curve showed a significant correlation and indicate that this measure has great potential to be used as a tool to control the thermal treatment of aging.

Novel bake hardening mechanism for bainite-strengthened complex phase steel

In the automobile industry,stamping and paint baking processes are used to strengthen components,and this not only saves costs,but also further reduces the bodyweight of automobiles.In this work,the bake hardening mechanism of the complex phase(CP)steel CP980 was explored by comparing it with that of DP1180,which has a clear bake hardening mechanism and a carbon content similar to that of CP980.By analyzing the bake hardening response and the microstructural changes of the two steels,we found that the bake hardening process of CP980 was divided into three stages.In the first two stages,the carbon atoms diffused into dislocations to form Cottrell atmosphere pinning dislocations,and excess car-bon atoms formed carbon clusters or low-temperature carbide pinning dislocations that were similar to DP1180.In the third stage,the dislocation acted as rapid channels for carbon diffusion,and fine cementite gradually formed when the carbon clusters gathered at the dislocations as precursors,resulting in pre-cipitation hardening.This novel bake-hardening(BH)mechanism is crucial for obtaining a comprehensive understanding of the strain-baking behavior of advanced high-strength steals(AHSS).

Improving the Mechanical Properties of Mg–Gd–Y–Ag–Zr Alloy via Pre‑Strain and Two‑Stage Ageing

In this study,we investigated the effects of single-stage ageing(SSA),two-stage ageing(TSA),2%pre-strain+single-stage ageing(P2%SSA)and 2%pre-strain-I-two-stage ageing(P2%TSA)on the mechanical properties of as-extruded Mg-8Gd-3Y-0.5Ag-0.5Zr alloy(E alloy).Compared with the SSA treatment,the TSA treatment increased the number density of/β'phase.The P2%SSA and P2%TSA treatments generated theγ'phase and chain-like precipitates in addition to the/β'phase.The contributions of these ageing treatments to the alloy strengthening can be ranked as P2%TSA>P2%SSA>TSA>SSA,because the increments in the tensile yield strength were estimated to be 199 MPa>148 MPa>144 MPa>110 MPa.Dif­ferent from the traditional strengthening of/β'phase in the E+SSA and E+TSA alloys,the composite precipitates compris­ing the/β'phase,γ'phase and chain-like precipitates in the E+P2%SSA and E+P2%TSA alloys provided better combined strengthening effect.The/β'phase was still dominated in the strengthening effect of the composite precipitates.Owing to the higher number density of/β'phase in the composite precipitates,the E+P2%TSA alloy exhibited the better mechanical performance as compared with the E+P2%SSA alloy.Finally,the E+P2%TSA alloy had the ultimate tensile strength of 452 MPa,the tensile yield strength of 401 MPa and elongation to failure of 3.3%.

DEVELOPMENT OF MATERIAL USED FOR AVIATION APPARATUS AXLE JOURNALS

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